1. Field of the Invention
The present invention is directed to a brake system having a tandem brake valve that is provided with a pressure equalization circuit and a force feedback circuit.
2. Description of the Prior Art
Four wheel drive loaders are articulated work vehicles having a forward mounted loader bucket for moving bulk materials. Four wheel drive loaders have a supporting structure comprising a front frame section having two front wheels and a rear frame section having two rear wheels. The front and rear wheels are all driven and are all provided with brakes.
The brake system for a four-wheel drive loader has a left pedal and a right pedal. Both pedals actuate the front brakes and the rear brakes. Typically when the left pedal is depressed, a clutch cutoff signal is sent to the transmission declutching the transmission. The operator can then use his right foot to depress the accelerator pedal accelerating the engine and increasing hydraulic fluid flow to the work implements. Such a two-footed operation reduces cycle time. Some operators prefer to operate the loader with only the right foot.
These brake systems may comprise parallel brake circuits for the front and rear brakes. The flow of pressurized hydraulic fluid through these circuits may be controlled by the tandem brake valve. The tandem brake valve has a first flow regulating portion to control the flow of pressurized hydraulic fluid to the front brakes and a second flow regulating portion controlling the flow of hydraulic fluid to the rear brakes. Each of the brake circuits may be provided with a hydraulic pressure accumulator to maintain pressure in the circuits even if the source of pressurized hydraulic fluid fails. One problem with these systems is the unequal pressure in the brake circuits causes unequal wear of the brake components. For example, the front tires and brake pads may wear more quickly if the hydraulic pressure in the front brake circuit is consistently higher than the hydraulic pressure in the rear brake circuit.
A work vehicle having a work implement is provided with a supporting structure having front wheel and rear wheels. The work vehicle is provided with a brake system having a left pedal and a right pedal. The brake system comprises two parallel brake circuits for controlling the front brakes and the rear brakes. The front and rear brakes are triggered by depressing the right or left brake pedal. The first brake circuit has a first brake actuator, a first hydraulic line, a first flow regulating portion of a tandem brake valve and a first accumulator. The second brake circuit has a second brake actuator, a second hydraulic line, a second flow regulating portion of the tandem brake valve and a second accumulator. The first brake circuit controls the application of the front brakes and the second brake circuit controls the application of the rear brakes.
To equalize the hydraulic pressure applied to the first and second brake actuators, an equalizer hydraulic line extends between the first and second hydraulic lines. The equalizing hydraulic line is provided with an orifice for limiting hydraulic fluid flow between the first and second hydraulic lines if one of the lines fails.
A force feedback hydraulic circuit is hydraulically positioned between the first and second hydraulic lines. The force feedback circuit comprises a first force feedback hydraulic line, a second force feedback hydraulic line and a force feedback hydraulic actuator. The first feedback line extends between the first and second hydraulic lines. The second feedback line extends from the first feedback line to the feedback actuator. The feedback actuator is mounted to the tandem brake valve and provides a feedback force to the operator by resisting the depression of the pedals. The first feedback line is provided with a shuttle check valve having a first inlet, a second inlet and an outlet. The first inlet is in fluid communication with the first hydraulic line and the second inlet is in fluid communication with the second hydraulic line. The outlet is hydraulically coupled to the second feedback line.